1. Field of the Invention
This invention relates to a noncontact type tonometer for measuring the intraocular pressure of a subject's eye.
2. Description of the Prior Art
Heretofore, a noncontact type tonometer is known in which a detector detects alignment between an optical axis of a main optical system of the instrument and a visual line of a subject's eye and determines whether a working distance between a cornea of the eye and the instrument is within an allowable range or not, and then the measurement of the intraocular pressure of the eye is automatically performed based on a result obtained by the detector.
In such a tonometer, alignment is first performed between an optical axis of an anterior eye portion observing optical system and a visual line of the eye, and then pulsed air is discharged toward the cornea of the eye. The cornea is transfigured by the discharge of the pulsed air. According to the corneal transfiguration, the quantity of light reflected by the cornea increases, and thereby the quantity of light received by a light receiving sensor increases. Therefore, the intraocular pressure can be measured by measuring an increase in the quantity of the light received by the light receiving sensor.
According to the conventional tonometer, when measuring the corneal transfiguration, the pulsed air scatters tears or the like from the eye. And cases frequently occur in which part of the scattered tears adhere to optical members, such as a cover glass, an objective lens and so on, facing the eye. If the tears adherent to the optical members are not removed, they become dry and, as a result, elements other than water of the tears or fine dry particles mingled with the tears are left thereon and become stains or spots.
Therefore, as a way of finding such stains, a lens cap with a reflection plate is removably mounted in front of the optical members. A judgment on the stains is formed such that the quantity of alignment light reflected by the reflection plate is detected and compared with that of alignment light detected when the optical members have no stain.
However, in a case where a judgment whether there is any stain is formed by such a lens cap, the following faults are developed.
1 If the lens cap is mounted obliquely, the reflected alignment light for alignment cannot be received accurately, and therefore a high precision in mounting the lens cap in the instrument is needed.
2 In spite of the fact that the reflection plate of the lens cap is stained, cases occur where the stains on the lens cap are regarded as those on, for example, the objective lens.
3 Since a measuring process for the corneal transfiguration is different from an ascertaining process for the stains, there lies the trouble.
4 If a stain check is not performed at regular intervals of time, the measurement of the corneal transfiguration might be performed in a stained state.
5 If an arrangement is adopted in which a fixation target on which the eye is fixed is looked by the subject through the discharging nozzle, stains on the aforementioned optical members cannot be distinguished from stains on optical members located after the discharging nozzle which are made by an absorbing action of the nozzle after discharging the pulsed air.